1. Field of the Invention
The present invention relates generally to a method and device for detection of intermodulation distortion (IMD) and reduction of IMD in a code-division-multiple-access (CDMA) mobile telephone system.
2. Description of the Related Art
The rapid growth rate of cellular mobile telephone subscribers, the need to use the cellular phones not only for voice, but also data transmission, and the limited capacity of analog cellular systems, is driving the implementation of digital cellular systems to provide increased capacity and higher quality signals. At some point, a complete transition from analog to digital systems may occur. However, until that time, the two systems must co-exist, and, since there will be no additional spectral allocation for digital cellular, the digital and analog cellular systems must share the same spectrum--869-894 MHZ for receive and 824-849 MHZ for transmit signals.
A digital cellular system using CDMA (code-division-multiple-access) technology has several advantages over other proposed digital technologies, i.e., TDMA and FDMA (time- and frequency-division multiple access, respectively), including ease of frequency planning, increased capacity (at least theoretically) and improved handoffs, among others. CDMA systems have been introduced into high-density markets, i.e., large cities, where analog cellular systems using the AMPS (advanced mobile phone system) are already well established. In these areas, the CDMA base stations are sparsely interspersed amongst a number of existing AMPS base stations. Depending on the distance between the CDMA mobile station (the CDMA cellular subscriber) and the CDMA base stations, and the relative proximity of the AMPS base station, the signal from the AMPS transmitter can generate significant interference which can overload the CDMA phone. This occurs because, when the sensitive amplifier at the receiver front end is driven with the high level of the AMPS signal, it operates in the non-linear region of the amplifier transfer function. In the non-linear operating region, frequency components mix to create new frequency terms in addition to the signals in the received spectrum. This interference, a significant component of which is intermodulation distortion (IMD), occurs when a CDMA phone is physically located far from a CDMA base station, but near an AMPS-only (not co-located) base station. IMD which occurs in this situation has been labeled "mobile-generated IMD".
When mobile-generated IMD (hereinafter, simply "IMD") occurs, two or more signals mix within the receiver due to the non-linearities of the device when operated at overdriven power levels, producing IMD product signals at frequencies that sometimes fall within the receiver passband. If an IMD product within the passband is at a sufficient level relative to the CDMA signal strength, it may interfere with the desired receive signal. Since the CDMA band has a receiver bandwidth that is forty-one times wider than an AMPS band, the probability of an IMD product falling within the CDMA band is relatively high. This creates a mechanism where the narrow band system, including AMPS, NAMPS (Narrow band AMPS), or IS-136-based TDMA, causes a pulse-jamming type of interference over the wide band system, such as IS-95-based DS-CDMA (Direct Sequence CDMA).
The IMD generally occurs in the active stages of the mobile phone receiver front end, between the antenna and the channel selectivity filter. FIG. 1, labeled "Prior Art", is a block diagram of a typical active stage for a CDMA phone receiver front end. An active stage 100 typically includes an RF low-noise amplifier ("RF LNA") 102, a mixer 104 (which is coupled to a local oscillator), and an IF amplifier 106, which receive the signal via duplexer 108. The interfering signals pass through the front end RF filter 110 and are not blocked until they reach the channel selectivity IF filter 112. By that point, the IMD products have already been generated and are located within the passband of the DS-CDMA signal.
The practical intermodulation spurious response attenuation for a standard IS-95 compliant mobile phone is approximately 58 dB. This level of attenuation provides reasonable power consumption at a cost consistent with the requirements of a cellular phone targeted to a mass market. However, field tests conducted on trial CDMA/AMPS dual-mode networks have shown the 58 dB attenuation to be inadequate. FIG. 2 illustrates an exemplary relationship between AMPS signal power levels (along the horizontal axis) and desired CDMA power levels (along the vertical axis), with line 20 indicative of the 58 dB intermodulation interference tolerance margin. The 58 dB line corresponds to a minimum carrier-to-interference ratio (Ci/l) required to maintain acceptable voice quality when the network is highly loaded. The points below the line indicate degraded voice quality and an increased call drop rate, with the Ci/l increasing above the line. Actual field test measurement points are shown in the area designated by reference numeral 22, with all measured points being well below the line, indicating a significant problem for IS-95 based CDMA phones.
One possible solution to providing CDMA service in areas where there are high levels of interference is to co-locate the CDMA cells with the AMPS or other narrow-band cells. This remedy is impractical, particularly for the initial introduction of CDMA into established AMPS territories because a one-to-one match-up of base stations would be very costly. Because there are so many AMPS base stations, the CDMA system will still be vulnerable to system interference due to a near-far effect when the CDMA subscriber is in the vicinity of other noncolocated AMPS sites. Another possible solution is to improve the immunity of the receiver to interference by increasing the dynamic range capability of the LNA, specifically increasing the maximum power handling capability. However, this solution is also impractical because it requires increasing the supply current to the receiver and defeats one of the significant advantages of CDMA--that of increased battery life and greater talk and stand-by time in the cellular phone. Since these solutions cannot remedy the interference problem without substantial compromise, efforts have been directed to minimizing the IMD interference.
IMD may be defined in terms of the peak spurious level generated by two or more tones injected into a receiver. A solution to minimize the effects of IMD is to attenuate strong IMD (IIP.sub.3 (third-order intercept point)) signals at the mobile station's front end, i.e., between the antenna and the active elements. The third order intercept point is typically defined for a receiver as the input power (in the form of two tones) required to create third order distortion products equal to the input two tone power. The higher the IIP.sub.3, the lower the level of IMD products generated for a given level of input tones. Because the low noise amplifier (LNA) has a low IIP.sub.3, and because it is needed for providing the target noise figure when receiving the weak desired signal, it is logical choice for the point of selective, i.e., switched, bypass and/or attenuation. The LNA is a wide bandwidth step amplifier with a step size that may typically vary between 15 and 22 dB, depending on the design partition, although it can cover a range anywhere from 0 dB on up.
Different methods have been proposed to reduce the level of IMD products, including that described by Bain in his article entitled "Reducing IM distortion in CDMA cellular telephones", published in RF Design, Dec. 1996, pp. 46-53, which is incorporated herein by reference. Bain describes a method for determining the RF input level for a given signal and switching in an attenuator prior to the LNA if that level exceeds a predetermined threshold. The LNA is not bypassed, but remains active within the receiver front end. The attenuator can be fixed or variable. Since the threshold and detection are both based upon the total received signal level without regard to the AMPS signal level, signal-to-noise ratio (SNR) is sacrificed, and the risk of a call being dropped is high if the AMPS signal level is much larger than the CDMA signal level. In this situation, the combined signal is high enough to switch in the attenuator, but the relatively weak CDMA signal is of such a low level that, without amplification, it cannot be detected, resulting in a loss of the call.
Another method is described in the international patent application of Wheatley, et al. (International Publication No. WO 96/19048; International Application No. PCT/95US/16002, entitled "Method and Apparatus for Increasing Receiver Immunity to Interference"), which is incorporated herein by reference. This method uses a pair of switches for coupling the received signal to either an amplifier input in the first switch position or directly to a bandpass filter, bypassing the amplifier, in the second switch position. A microcontroller monitors the power of the received signal, switching between the first and second positions in response to the total received signal power exceeding a predetermined threshold. An alternate embodiment continuously adjusts the front end gain based upon another predetermined power threshold and information from an included received signal strength indicator (RSSI). As in the method proposed by Bain, the activation of the attenuation is based upon the total received signal level, without reference to the relative contribution of the undesired AMPS signal (or other source of interference). Therefore, the risk remains that low level CDMA signals will be dropped because the attenuator setting is too high or the received signal amplification is too low.
Efforts toward providing more specific reaction to IM interference to avoid unnecessary activation of an amplifier bypass are hindered by the fact that IM interference is a random process, making detection of the interference in a mobile channel environment a difficult task. FIG. 8 provides an example of the presence of IMD across the CDMA bandwidth as determined from the output of an FFT processor. This plot, which was generated using field measurements conducted while traveling in the direction of an AMPS base station, illustrates how the interference appears in small bandwidths within the total bandwidth, and how it is randomly spread across the bandwidth. When the entire bandwidth is used to detect IMD power level, the effect of the narrow-band interference cannot be properly taken into account.
Similar interference from IMD may be experienced in broadband PCS 1900 (personal communications services) wireless phones due to the use of low noise amplifiers in the receiver front end and the required simultaneous reception of all cellular channels. The PCS band is at 1930-1990 MHZ for receive and 1850-1910 MHZ for transmit signals, therefore, there is no overlap of AMPS signals within the same input spectrum. However, since IMD is a product of mixing of two or more input signals within the low noise amplifier, a high level relatively narrow-band signal from a nearby AMPS base station or other source could, nonetheless, result in IM products within a PCS 1900 mobile phone, resulting in degraded signals and dropped calls.
For the foregoing reasons, in order to effectively integrate CDMA- and other broadband-based mobile phone systems into AMPS service areas, the need remains for a method and system for detecting AMPS-induced IMD interference for selective activation of an IMD product-reducing operation which minimize the incidence of broadband-based calls that are dropped due to over-compensation or lack of sensitivity in the IMD filtering mechanism. A successful method for determining whether IMD interference is present requires consideration of the exact nature of the interference. The prior art solutions have failed to make such a consideration.